1. Field of the Invention
This invention relates to liquid toners that are useful for electrographic and electrophotographic processes.
2. Discussion of the Art
A general discussion of color electrophotography is presented in R. M. Schaffert, Electrophotography, Focal Press, London & New York, 1975, pp. 178-190. Electrophotographic systems are systems in which a toner is deposited on a charged surface and subsequently transferred to a receiving sheet. Electrophotographic systems employing liquid toners are well known in the imaging art. See, for example, Schmidt, S. P.; Larson, J. R.; Bhattacharya, R. in Handbook of Imaging Materials, Diamond, A. S., Ed., Marcel Dekker, New York, 1991, pp. 227-252 or Lehmbeck, D. R. in Neblette's Handbook of Photography and Reprography, Sturge, J., Ed., Van Nostrand Reinhold, New York, 1977, Chapter 13, pp. 331-387. A liquid toner is a dispersion of small, e.g., colloidal particles in a dispersing medium having a low dielectric constant. The particles usually comprise a colorant, typically a pigment, and a film-forming resin and carry an electrostatic charge. The particles in the dispersion are capable of migrating under the influence of an electric field and being deposited on a surface bearing an imagewise distribution of opposite charge, thereby forming an image.
In most instances, the preferred dispersing medium has been a hydrocarbon having a high flash point, which hydrocarbon has both a low dielectric constant, e.g., less than 3, and a vapor pressure sufficiently high to ensure rapid evaporation of the dispersing medium following deposition of the toner onto a photoconductive drum, transfer belt, and/or receptor sheet. Rapid evaporation is particularly important for cases in which multiple colors are sequentially deposited and/or transferred to form a single image. Examples of such commercially available dispersing media include members of the family of solvents having the trade designation "ISOPAR" (boiling point range: 130.degree.-160.degree. C.).
There are significant drawbacks to the use of toners employing hydrocarbon dispersing media. These drawbacks include (a) adequate evaporation rates for high speed imaging applications, (b) low flash points (hydrocarbon solvents having boiling points below 120.degree. C. typically have flash points below 40.degree. C.), (c) environmental pollution, and (d) toxicity. Chlorinated hydrocarbons, a subgenus of hydrocarbons, are undesirable from the standpoint of atmospheric pollution. It would be advantageous to employ a dispersing medium having a higher evaporation rate, lower pollution effects, lower flammability, and lower toxicity than those of ordinary hydrocarbon solvents. Chlorofluorocarbon solvents (e.g., "FREON-113" solvent) have been employed as dispersing media for electrophotographic liquid toner dispersions. See, for example, Soviet Pat. No. 1,305,623. However, chlorofluorocarbon solvents of this type are likely to be banned because they are believed to cause ozone depletion in the stratosphere, on account of formation of chlorine monoxide, a free radical that is capable of destroying ozone.
One class of solvents that addresses some of the aforementioned problems are highly fluorinated, preferably perfluorinated, solvents, such as the "FLUORINERT" solvents (available from Minnesota Mining and Manufacturing Company), hexafluorobenzene, and so on. These solvents have many desirable physical properties that make them useful in electrophotographic applications employing liquid toner dispersions. However, they suffer from the shortcoming of being unable to dissolve or disperse most materials, including hydrocarbon-based materials. Thus, in order to develop an electrophotographic process employing highly fluorinated solvents, it is necessary to develop stable solutions or dispersions of colorant and charging agents. This can be accomplished by preparing polymers that are capable of being dispersed in those solvents that are also capable of dispersing dyes or pigments, and dispersing those polymers and dyes or pigments in those solvents, as described in U.S. Pat. No. 5,283,148, incorporated herein by reference. The fluorinated solvents described in that reference include perfluorinated species of alkanes, ethers, arenes, alkarenes, aralkanes, alkenes, and alkynes. The solvents may contain rings. Non-limiting examples of fluorinated solvents include perfluoroheptane, perfluorinated 2-butyltetrahydrofuran and mixtures thereof with perfluorooctane, perfluorohexane, perfluorotributylamine, perfluorotriamylamine, "FLUORINERT" solvents available from Minnesota Mining and Manufacturing Company, such as "FLUORINERT" solvents having the designations FC-84, FC-77, FC-104, FC-75, FC-40, FC-43, FC-70, FC-71, etc. It is recognized that many perfluorinated materials have residual amounts of hydrogen atoms that were not replaced by fluorine; however, it is anticipated that hydrogen atoms in the solvent are not deleterious provided that the total fluorine content of the solvent is greater than about 60 percent by weight.
It is known by those skilled in the art of colored electrophotography that both dyes and pigments have been used as the colorant in toners. One of the advantages of pigments is that migration, or "bleeding", is minimized at the fusion step. The primary advantages of dyes are their bright colors and transparency. Toners comprising polymeric dyes provide the advantages of being both highly transparent and non-migratory, which make them well-suited in applications requiring high quality images, such as, for example, in proofing or business graphics. Another advantage of using toners comprising polymeric dyes over toners containing pigments is that the former have the potential to be more conformable to the final image receiving layer. Conformability is particularly important when the receiving layer is plain paper, where abrasion resistance and adhesion are important considerations. A further advantage of toners comprising polymeric dyes over toners containing pigments is greater stability of the dispersion, because flocculation, caused by desorption of the colorant from the toner, is not likely in these systems. A process advantage of toners comprising polymeric dyes is that the milling operation required to incorporate a pigment in a dispersing medium is avoided.
Polymeric dyes can be classified in two classes: (1) backbone polymeric dyes and (2) pendant polymeric dyes. In backbone polymeric dyes, the dye moiety is a segment in the polymeric chain. In pendant polymeric dyes, the dye moiety is tethered to the polymeric chain either directly via the dye moiety or indirectly via a connecting group, e.g., an alkylene group. A typical backbone polymeric dye can be prepared by reacting a dye containing two reactive groups, such as two acid chloride groups, with a colorless organic diol or diamine. Many backbone polymeric dyes are based on polyesters or polyamides. There are two general methods for preparing pendant polymeric dyes: (1) the polymerization of a monomer containing a pendant dye moiety, and (2) the reaction Of a pre-formed polymer with a reactive dye or reactive dye developer.
It would be desirable to develop a polymeric dye suitable for a toner that is dispersible in a highly fluorinated solvent.